High altitude long endurance aircraft require extremely efficient designs. Hydrogen-powered aircraft have been previously suggested to limit fuel weight and thereby maximize endurance. Regardless of the selected fuel, an oxidizer must be provided, and if ambient air is to be the oxidizer, it typically must be compressed at high altitudes. When hydrogen is the fuel, a significant amount of compressed air may remain unburned by an engine, and thus the energy of compression may be wasted for that unburned portion of the air.
It has been suggested that the use of electric motors for propulsion may be beneficial for high altitude long endurance aircraft. If a high altitude long endurance aircraft is to operate electric motors, a significant electrical power generation system is necessary. Such systems must be able to quickly adapt to changing power requirements, but complex systems with heavy components are detrimental, in that they limit the payload (or duration) of the aircraft, and typically have lower reliability. Simpler systems, on the other hand, may be limited in their ability to adapt to rapidly changing power requirements.
Conventional turbocharged engines are usually designed to control boost pressure using a wastegate. A wastegate is a controllable valve in the exhaust stream that bypasses some fraction of exhaust gases pass a turbocharger, thereby providing control over the turbocharger speed and resulting compressor boost. The continuous use of a wastegate can provide for the prompt ability to boost power (by closing the wastegate), but use of a wastegate wastes some of the energy that otherwise would be recoverable from the exhaust. In a high-altitude hydrogen powerplant, it is important to optimize efficiency, so a turbocharger system is typically set out to operate normally with zero wastegate flow. Because of this, it is not possible to increase boost by further using a wastegate.
Accordingly, there has existed a need for an aircraft powerplant that can provide highly efficient power with high reliability, while allowing for rapid changes in operating levels. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.